Purpose The aim of this study was to determine the impacts of reduced aboveground and belowground C inputs on the community composition of soil microbes and enzyme activities in a seasonal context.

Materials and methods Litterfall removal, root exclusion, and stem girdling treatments under a subtropical conifer plantation growing on a coarse texture of sandy soil in southeast China were employed. One year after the initiation of the treatments, we measured the soil microbial biomass, community composition, and enzyme activities, including hydrolytic and oxidative extracellular enzymes on a seasonal basis. Soil inorganic N, dissolved organic C and N, and available P were also determined.

Results and discussion Seasonal variations of soil microbial composition and enzyme activities were attributed to soil temperature and moisture and soil nutrient availability. Girdling treatments significantly increased the abundances of gram-negative bacteria and actinomycetes in winter when soil temperature, moisture, and available nutrients were at the lowest level among the four seasons. Girdling alone and girdling combined with litter removal and root trenching significantly decreased the cellobiohydrolase, β-glucosidase, β-1,4-N-acetylglucosaminidase, and acid phosphatase activities in autumn. These hydrolytic enzyme activities were significantly correlated with soil moisture, NH4+, DOC, and available P. We also found a significant relationship between hydrolytic enzyme activities and the ratio of gram-positive to gram-negative bacteria.

Conclusions Plant belowground C allocation, soil temperature, and moisture drove the seasonal patterns of soil microbial composition and enzyme activities. Labile C input by root exudates is a key determinant of ecosystem functions mediated by soil microbes such as microbial decomposition processes.